Mic transformer load question

Hi all

I need to select a Mic input transformer for a pre I'm doing.

In simulation, the input Z of the pre with no transformer is 38.4kOhm. So, if I select the Z of the mic as 200 ohm to match to, I'd get a turns ratio of 1: 13.8.

My question is, is 200 Ohm the right figure to use? For e.g. Neve pre's want to match to 300 or 120 Ohm mics, what is the rationale behind this?

Regards
Peter

You'd only use a 1:13.8 input transformer, in this case, if you wanted the mic to be working INTO 200 ohms, which is definitely not good. The mic has a source impedance of about 200 ohms, and should work into a load of about 2K, or higher.

With the primary terminated with 200 ohms--the nominal source impedance of the mic--the reflected impedance at the secondary should be 1/10th of whatever load you're connecting it to, in this case the preamp input. So, let's say the secondary should be about 3K. A 1:4 input transformer should do nicely (with its 1 to 16 impedance ratio).

As for different figures for "typical" mic source impedance: it's around 150-200 ohms for most professional mics. The usual practice in the USA was to call it "150 ohms", and in Europe, "200 ohms", but it's all an approximation anyway.

What would be the ramifications of using a 1:1 transformer, and letting the mic work into the full 38K load? Isn't a higher load going to load the mic less, and that is a Good Thing? Or is there some reason we want that 10% voltage drop, for a certain current draw? or what...

This has been a constant unclear spot for me

Also, Gus helped me discover that my U87 was strapped at 50ohm output, and told me that strapping it at 200ohm would increase the output. I'm not sure I understand why this is!

[quote author="tmbg"]This has been a constant unclear spot for me [/quote]
Me too! Even a talented EE + professor who first explained to me the aforementioned 1:10 Z matching rule could not say why it shouldn't be any higher. Can you unconfuse us, New York Dave?

art of electronics always talks in terms of "make sure Zin is AT LEAST 10 times greater than Zout" which is why it sticks in my head that way. I've never seen any sort of argument why a higher Zin and lower Zout would be undesirable.

EDIT: except in the case where loading is the desired effect, such as when I was building R2R dacs for video, and I wanted the end result to load 3.3V down to 0.7V by the 75ohm termination of VGA

Peter, what kind of mic, or numerous types, are you going to be using?

Limiting factors for not using anything above 1:10 is usually just capacitance.
That's why the 1:30 V76 input has a seven section bobinn, which reduces capacitance.

To uncomplicate the mic input, forget about the secondary side for a while. Just think of the primary first. Most microphones do not have a ton of current they can deliver. So you need an minimum amount of inductance, or henries, to keep from losing the small signal. This inductance can just be seen as the ac resistance the input iron is going to offer the mic.This will change acording to frequency.
The lowest henries I have seen for a mic input was about 2 henries for the DuKAne 3A55A. The highest I have seen is 16 henries on a light green UTC A-10. Typical A-10 impedance is about 10 henries. The Lundahl 7903 comes in with a primary inductance of abot 5 henries. So throw out the high and the low and you can say you want about 5 to 10 henries of inductance for a mic input. To file this away for instant retrival, use A-10 = 10 henries as a pneumonic device to figure mic input L.

Use the typical 1000 cps standard and this works out to about 6 * 1000 * (5-10) which is 6000 * (5-10) = 30,000 to 60,000 ohms. Plenty to keep the millivolt mic signal from deteriorating. Use 100 cps for your calcs and you come up with 3000 to 6000 ohms. Ahh, there's that magical "the mic should see a min load of ten times.. " bussiness.

Saturation really isn't an issue with mic inputs, so you can use some nice 80 percent Ni to get your inductance without all the turns (think capacitance).

Now, you want to get off the noise floor as soon as possible with a mic signal, so something better than 1:1 ratio is usually used, although there would be nothing wrong with using a 1:1 transformer as long as you had enough primary inductance and clever enough circuitry to amplify the signal without all the noise.

So, we have the primary figured out, and we know we want some voltage gain. The only thing left to complicate us is what the mic will be seeing as a first stage. Usually this is not an issue as a tube grid has plenty of input impedance that will not load down the primary. A transistor input might be a different story. This is why you see a lot of fet's being used in the discrete opamps.

This is how I look at it. Probably an over simplification, but if I want a bunch of gobbly goop, I will open up RDH4 or get on the internet.

Also, most mic pre's want to cover a certain amount of range of mic impedances, so they use the standard 600 ohm thing and usually deviate a couple of hundread of ohms above and below, thus the 300-12000 ohm thing you see with the Neve stuff.

Thanks for the lesson, CJ!!!

Can you unconfuse us, New York Dave?

Click to expand...

To uncomplicate the mic input,

Click to expand...

2005 looks promising - I like these friendly words that bring a smile to your face :grin:

ok i'm still a little foggy on the mic impedence thing.. when you say 600(insert other number if you need)ohms is that measured from signal-/+ to ground or from signal- to signal+? Also, my preamps measure 3.3k to ground on both signal inputs, I've lowered the value to 600ohms and it improved the sound, when you say you increase the numberical value to lets say 10k number aren't you really LOWERING the impedence that the mic sees and vice versa?

Usually between signal- to signal+.

when you say you increase the numberical value to lets say 10k number aren't you really LOWERING the impedence that the mic sees and vice versa?

Click to expand...

If I understand you correctly: higher numbers increase impedance but lower the load ('strain').

ok thanks that clears it up for me. for some reason i always thought that you measure this to ground..

as for resistors from the signal to ground, what would this be considered and what does it do exactly? i'm not sure i've measured from signal- to signal+ on my preamps.. they are BJT input and there is nothing between the signals like resistors or caps. does it usually show a reading in this fashion, from base to base?

Don't confuse dc resistance with impedance,. They are two different things.

Impedance is a combination of reactance and resistance.

Although impedance can inviolve complex numbers (j operators), the average Joe can use a very simple technique to get a close number.

In the case of inductors, plot reactance at a given frequency, say 1000 cps, on the x axis of a graph, and plot dc resisyance on the y axis.

The impedance of the system will be the resultant vector magnitude formed by the two quanities. You can even get phase shift from the angle of the resultant.

Here is an example of how to calculate impedance, which really is not used that much in transformer analysis because usually the dc resistance is low enough not to make much difference.

Take a 1 henry inductor with 1000 ohms of dc resistance. (terrible, but just for illustration). Lets analyze impedance at 1000 cps. 1 henry times 6.28 times 1000 equals about 6 k ohms. Plot that on the x, and plot 1000 dcr on the y. The resultant is 6.08 k ohms. from sq triangle formula a^2 + b^2 = c^2 ) The angle that the impedance vector makes is about 9.46 degrees. This would be the phase shift of the circuit.

An A-10 transformer only has about 70 ohms of dcr, so it really dosen't make that much difference to the impedance or phase shift.

Nothing too mysterious--I just assumed that Peter wanted to use a mic input transformer for the usual reasons: to present a higher source impedance to the amplifier input while obtaining some voltage gain. If the amplifier in question gives good low-noise performance when driven from a 200-ohm source, and if you don't need to step up the signal voltage at the input, then by all means use a 1:1 transformer... or no transformer at all.

Google on the subjects of "optimum source impedance" and "noise resistance" if it's unclear why on earth one would want to increase the source impedance seen by an amplifier input. I admit that it seems odd at first glance, when you consider the usual dictum of keeping source impedances as low as possible. Here's some reading to get you started:
http://mems.caltech.edu/courses/EE40%20Web%20Files/Noise%20in%20Systems.pdf
...or see the quote at the bottom of this message for a less-mind bending summary.

Yes, the mic will deliver its highest possible voltage output into the highest possible impedance; but most microphones are spec'ed for flat response into a certain defined range of load impedance, which is usually on the order of 2000 ohms or so. Going higher may cause peaks in the response, going lower can cause dips or rolloffs. This is the reason why the character of a mic can seem to change as you vary the load impedance.

Paul Stamler wrote:

As Bob says, the tube doesn't really care. But it will give its best noise performance when the source resistance feeding the grid is at least 3.85x the total equivalent noise resistance of the tube and its associated
components. (For this exercise I define "best noise performance" as adding less than 1dB of noise to the inherent noise of the microphone, which is assumed to be a 150-ohm resistor.)

That total noise resistance has two or three components:

1) The inherent noise resistance of the tube. In a triode, this
(oversimplifying) is typically 2.5/gm, where gm is the transconductance of
the tube at its actual operating point, in mhos -- excuse me, Siemenses.

2) The plate resistor, divided by the square of the tube's gain.

3) The cathode resistor, if unbypassed.

Add all these resistances up, and you get the equivalent noise resistance.
So for a tube with a gain of 10x, a plate resistor of 50k, a cathode
resistor of 2.2k, unbypassed, and a gm of 2 milliSiemens, you get an
equivalent noise resistance of:

[50k/100] + [2.5/0.002] + 2200 = 500 +1250 + 2200 = 3950.

3.85 x 3950 = 15,207; in practice, a transformer with a secondary of 15k or higher will do fine. The LS-series transformer used in the RCA preamp under discussion has such a secondary; so does the Jensen JE-115K-E, widely used in tube preamp designs.

Click to expand...

Why you use 1:10 loading:

100Ω source, 100Ω load = -6dB
100Ω source, 1,000Ω load = -1dB
100Ω source, 10,000Ω load = -0.1dB
100Ω source, infinite load = -0.00000000...dB

Since we can hardly hear a 1dB shift, 1:10 loading is about as good as it gets.

Since there is always some downside to a high impedance, we might go over 1:10 (hi-fi often works 220Ω source, 50,000Ω load) but we don't go crazy. If nothing else: when the source gets disconnected, a too-high input impedance sucks more hum and buzz from thin air.

> the input Z of the pre with no transformer is 38.4kOhm.

Who the hell cares? What is the noise impedance? (SPICE can tell you, but it is like reading tea-leaves.... if you post a sketch of the first stage with bias I can probably guess at a glance, and maybe explain my process, unless my damm CD project goes off track AGAIN....)

Here's my sophomoric question:

What about impedence matching for equal power and voltage gain? Why is it that voltage gain is valued more than power?

Thanks for all the replies!

PRR, I'll post the diag when I get home tonight.

Regards
Peter

[quote author="Ethan"]Here's my sophomoric question:

What about impedence matching for equal power and voltage gain? Why is it that voltage gain is valued more than power?[/quote]
Exactly microphones are designet to pressure - to voltage transfer.
Basic damping {of transducer} is done at mechanic - acoustic side.
That, what we do by 2k load impedance of mic is damping of ringing
of parasitic coil leak inductance.
That 200 Ohm load is parameter of the dynamic mic.
Yes, it is possible to damp microphone at the electrical side,
by loading of the mic amp with very low {or theoretically zero}
impedance. But design of that preamp is complicated {there must be used
paralel feedback to do noise and power matching at the some time}, and
microphones, which want electrical damping are not commertionally
aviable {altough this system can have theoretically lower self noise}

There is not reason to power match of modern microphones.
If you are interested to theoretical design of {resistance controlled}
dynamic pressure mic controlled via electrical way {by some bridge hell, as used on RCA? film recording system}, you can do it. It is nice theme for thesis.

But there are other method of high quality recording that coil-type pressure mics. I mean, that there is very very small improvement to
coil type mic via electrical damping. That transducer can not be better,
than modern ribbon or condenser.

xvlk

[quote author="Ethan"] Why is it that voltage gain is valued more than power?[/quote]
This is historical question. At audio frequencies
with tubes or FETs we have only input noise resistance
{=input voltage}

With bipolars, there is {noise} optimal input resistance
It is 1:3 input trafo for normal BJT.

But in the BJT there are optimal input noise resistance higher than
power matching.

[quote author="Ethan"] Why is it that voltage gain is valued more than power?[/quote]
Simply, noise and power matching {resistances} are different.
And normally at low frequencies, they differs by ORDER.

Hi PRR

Here is the mic pre input cct.
http://www.groupdiy.com/index.php?topic=44828.0
Peter

Input is BJT biased at about 0.6V/6K8= 0.073mA.*

1) The emitter impedance is about 28Ω/0.073 or about 380Ω. This acts in series with the source. We want to transform the source to much higher than 380Ω.

2) Assuming Hfe is 100, ignoring feedback and bias, the shunt impedance is 38K, and we want to transform the source to much less than 38K.

In fact we have another 40K of shunt resistance. So the source should be transformed to much less than 38K||40K= 20K.

Some of your shunt feedback resistors (which act in series with the source) are much higher than 380Ω, but the highest-gain settings are very low Ω and I will ignore this for a moment.

Simply taking a geometric mean of 380Ω and 20KΩ suggests best noise performance around 2,750Ω. This does assume Hfe=100, and will shift a little with a large shift in Hfe.

What is the data-sheet claim for the actual NTE159 you propose? And since the NTE159 sheet is lean, what is the plot for any similar transistor? (I picked 2N3904 at random.)



The one-line noise spec for NTE159 happens to be very similar to the proposed operating condition. We don't know if this is the "perfect" condition, or just a handy test-jig, but it is reasonably low and probably good. Taking for granted that 1KΩ is "good", then we want to transform 150Ω to 1KΩ. We want a 1:2.58 transformer. Since the input impedance is really about 38.4K, the mike will see about 5.76K (in parallel with any Phantom resistors). This is much-much more than 150Ω so the mike will be lightly-loaded and we will see full output voltage.

How bad do things get if, instead of 0.1mA we run at 0.073mA, or can't find a 1:2.58 transformer? The 2N3904 data won't be exact-right, but the trends will be similar.

Indeed 0.1mA does seem to be as good as it gets, worsening slowly at other currents, and requiring a different value of source resistance for best noise performance. 0.073mA may be imperceptably worse than 0.1mA, but I doubt you can prove it on the bench, and this bias scheme favors low current in the input device.

For the 3904 working at 0.1mA, it seems that the best source impedance is actually 600Ω. For 0.050mA, about 1200Ω. So for 0.073mA we might guess 850Ω. Now we come up with a 1:2.38 transformer. BUT we can go as far as 250Ω or 2500Ω with a barely perceptable 1dB rise in noise figure. So transformers from 1:1.3 to 1:4.1 will all give "the same" noise. (Very different gain, but gain is cheap, which is why we hardly-ever want to match signal impedances.)

I have here a 150Ω:600Ω 1:2 transformer. After verifying that the unloaded input impedance really is above 150Ω at the lowest frequency of interest, I can use it and expect the noise to be within a few tenths of a dB of the very best I can get. This is so very close to optimum that we should consider transformer losses. A typical 150:600 tranny will add about a dB of resistance noise. I also have some 2K:10K trannies, a 1:2.24 ratio. which is "even closer" to the "ideal 1:2.38 ratio". But these have winding resistance in the hundreds of ohms, which will badly contaminate a 150Ω source with noise.

Look at the "obvious" 150Ω:38KΩ transformation. At 0.073mA the transistor noise figire is 14dB, awful bad. The voltage ratio is 1:16, so we get lots of cheap gain. Except the heavy load will drop the mike voltage to half of its rating: we really only get 1:8 of voltage gain. While this is 4:1 better than the 1:2 transformer, the noise figure sucks. Gain is cheap. Find another 1:4 of gain in the tubes/transistors (you probably have a ton of excess gain already).

I have ignored your feedback network. In the high-gain position, 82Ω is pretty negligible compared to the 850Ω optimum. But 910Ω is not negligible, and noise will rise. This may not matter much. If it does, if you have excess hiss at medium and low gains, you should reduce the 18K resistor (maybe as low as 2K) and scale the gain-set resistors accordingly (which will force the 220uFd much larger).

_*_ Screw-up! I missed the 68Ω resistor in the output emitter. So the input device current is more like 0.150mA, isn't it? Argh, I'm such a dumdum. No great matter: the noise curves of a high-Hfe BJT are so flat around optimum that even a 2:1 shift of bias current hardly affects the noise. You might prefer to aim for 300Ω source impedance for best result on paper. And it might be wisest to stick with readily available 1:2 150Ω:600Ω transformers because noise will not rise enough to measure. (also I suspect the 3904 curves are pessimistic about Hfe and leakage; that for a typical device the bottom of the noise curve runs to somewhat higher impedance with slightly lower noise at the bottom.)